1) Field of the Invention
The present invention relates to open-end spinning i.e. rotor spinning.
2)Description of Related Art
Open-end spinning machines generally consist of a series of individual spinning units, aligned on the two fronts of the machine, each of which consists of a spinning rotor, which produces twisted thread starting from the singularized fibres of a sliver, and a collection unit which—after controlling the quality of the yarn with the interpositioning at a slubcatcher between the two components—causes the yarn to be wound onto a tube to form a bobbin. This bobbin is thus formed by pulling and winding the yarn onto its surface, as it is pulled in rotation by the underlying roll on which the rotating bobbin in formation is resting. The yarn is spirally wound onto the rotating bobbin as the collection unit is provided with a thread-guide device which distributes the yarn with a backward and forward axial movement onto the outer surface of the bobbin.
The structure of the individual spinning station is illustrated in the scheme of FIG. 1, and its functioning is briefly described hereunder according to its normal operating mode.
Proceeding upwards, the single spinning station 1 consists of an actual spinning unit 2 and a collection unit 3, of which the main components which lead to the transformation of the sliver of parallelized fibres to the bobbin of wound yarn, are briefly illustrated below.
The feeding tape or sliver S is contained in a cylindrical vase 4 which is deposited with a double spiral. The sliver S is fed to the unit by a feeding roll 5 passing through the funnel-shaped conveyor 6 and reaches the card 7, a rotating roll equipped with a toothed washer which singularizes the fibres of the sliver S and sends them by suction to the spinning rotor 8, which operates in depression.
The singularized fibres are deposited by a centrifugal effect in the peripheral throat of the spinning rotor 8, which rotates at very high velocities (up to 150,000 revs/minute and over); from here they are collected and removed in the form of the thread F, axially leaving its central opening 9, receiving torsions by the rotation of the rotor itself in the course which runs between its internal throat and said opening 9, thus creating the twisted thread F.
The pulling of the thread is effected with a pair of opposite extraction cylinders 11 and 12 which seize the thread F and are driven at a controlled rate according to the arrow a, thus causing the linear production of yarn, generally indicated in m/min. The quality-control sliver 14 of the yarn F can be positioned before the cylinders 11/12.
The thread F thus produced enters the collection unit 3, passes through a thread-presence sensor 15 and encounters a compensator 16 for compensating the variations in length of the run between the spinning unit 2 and the depositing point of the yarn F on the bobbin. The thread-guide device 21 distributes the thread on the bobbin in formation by transversally moving with a backward and forward movement according to the double arrow b, activated by a motor 20 which drives a longitudinal rod 22 in common with the other units of the spinning machine.
The bobbin 25 collects the thread F and is held by the bobbin-holder 26 equipped with two idle and openable counterpoints 27 which become engaged with the base tube 28 of the bobbin. The bobbin in formation 25 is laid on its driving roll or collection cylinder 29.
Open-end spinning machines typically consist of a large number of open-end spinning units aligned on the two fronts of the machine, each equipped with driving units in common with the spinning units arranged on each front and in particular the organs cited above:                feeding rolls 5        card 7        spinning rotors 8        extraction cylinders 11/12        thread-guide device 21        collection cylinder 29        
Apart from the thread-guides 21, which are activated in an alternating backward-and-forward movement, the other organs are activated in rotation with motors in common by means of transmissions which run along the front of the machine and which transmit their movement to the rotating organ of each spinning unit.
The scheme of the movement transmission system—in a conventional open-end spinning machine—is described herein with reference to FIG. 2, with specific illustrative reference to the driving of spinning rotors 8 aligned on one of the two fronts of the machine, with the specification that the driving of the other rotating organs, for example the cards 7, can be analogous to that of the spinning rotors and that the present invention can also be advantageously applied for other rotating activations of open-end spinning machines.
With respect to other open-end spinning organs, the activation of the spinning rotors is that which has the most technical problems in view of the high velocity, power and tension values to which the transmission belt which activates the rotors of a whole spinning front, is subjected.
In the top-end of the spinning machine, the common driving units of the various organs of the single spinning units are positioned together with the drive and control organs of the spinning machine. As far as the spinning rotors are concerned, the supporting structure of the machine, not indicated in the figure, sustains the asynchronous electric driving motor 31, which transmits movement with the transmission belt 32 to the pulley 33, which is smaller and coaxial with respect to the main pulley 34, thus multiplying the linear velocity transmitted on the basis of the ratio of the diameters ø34/ø33. The driving belt 35 of the rotors winds the main pulley 34 by about 180° and reaches the idle counter-pulley 37. A rotation detector 38, for example with a probe disk, commonly called encoder, is situated on this counter-pulley, which allows the control unit of the spinning machine to detect the rotation rate of the pulley 37 corresponding to the linear rate of the rotor driving belt 35. On the basis of the values detected by the encoder, the control unit 39 of the machine controls and drives the asynchronous motor 31, to give the main pulley 34 the desired rotation rate, with a variable frequency current generator 40, commonly called inverter.
The belt 35 runs horizontally from the idle pulley 37 along the whole front of the spinning machine as far as the tail-end of the spinning machine with the upper branch of its run. Along its upper run the belt 35 encounters one or more idle supporting rolls 41 which keep it lifted to the desired level.
At the tail-end of the spinning machine, there are two tail levelling and counter-pulleys 43, 44 which allow the belt 35 to invert its run and return with the lower branch of its course defined by the counter-pulleys 44, 45.
On the lower branch of its run, the belt 35—as better illustrated in the enlarged detail—encounters the legs 47 of the spinning rotors, on which it rests tangentially and to which it transmits the rotation torque to said rotors, rotating them at the required rate, which can reach 150,000 revs per minute. On its lower run, the belt 35 also encounters a series of thread-tensioner guide pulleys 48, consisting of idler pulleys, opposite and slightly offset with respect to the rotor legs 47, which push the belt itself with a pre-established force F against said legs of the rotors.
Recently designed automatic open-end spinning machines are constructed for high productions by aligning on each front of the machine an increasing number of spinning units, reaching and exceeding two-hundred units for each front.
The front encumbrance of each spinning unit is in the order of 250 mm, as also the pitch s between the spinning rotors shown in FIG. 2. The installation—for example—of two-hundred units on each front leads to a length of the spinning machine of over 50 metres and with lengths of the transmission belt well over 100 metres, taking into account the driving and control top-ends which are envisaged for spinning machines and the necessary driving transmissions.
At current rotation regimes of the rotors, the performances required to the transmission belt are extremely severe. Its linear rate is in the order of 55-75 m/sec, its positioning tension from standstill is in the order of 700-950 N, the overall absorbed power per rotor is in the order of 120-180 W. A significant part of the power required can be attributed to the energy consumed for the flexural and tensional mechanical stress cycles to which the transmission belt is subjected in its run along its closed circuit: this energy results in the heating of the belt itself, in the reduction of its friction coefficient and transmittable power, in addition to a progressive deterioration in its mechanical characteristics. For these reasons, thin transmission belts are adopted, with a small transversal section and well stretched, having a rigidity which is as limited as possible to reduce the amount of energy dispersed for the flexure which is caused in their heating.
In its closed circuit movement, the transmission belt is less tense in its upper run and more tense in its lower run, along which it transmits the rotation torque to the legs 47 of the rotors and overcomes their resistance torque. In its circuiting, the belt 35 is periodically more or less tightened between the terminal pulleys.
The transmission belt 35 is already assembled with a considerable positioning tension, to ensure that during its run it is never slowed down, not even in its upper course. When operating, in its lower run the tension of the belt gradually increases to overcome the resistance torque of the rotor legs aligned along the machine. At each rotor activated, the tension increase on the belt is in the order of 2-4 N, and the resistant torque is in the order of 0,15-0,3 Nm, depending on the geometries and rates.
With an increase in the number of open-end spinning units aligned on each of the fronts of the machine, the power and driving torque to be transmitted to the driving system with the main pulley 34, consequently also increase within the approximately 180° of its winding on the part of the belt 35. With an increase in the number of spinning units, there is therefore a limit to the power and torque which can be transmitted with the main pulley, taking into account the flexibility and dimensional requisites typical for the driving of open-end spinning machines. Close to these limits, there is slippage and malfunctioning, especially when the friction coefficients between the belt and pulley begin to deteriorate.
Analogously, with an increase in the number of spinning units per machine front, the increase in tension of the belt between the tail-end pulley 44 and the main pulley 34 which drives the activation, is also greater. For 200 spinning units for each front of the spinning machine, the tension acting on the belt in correspondence with the main pulley 34 can reach values of even 1200-1500 N.
The driving system of open-end spinning machines according to the invention is defined in the first claim for its essential components, whereas its variants and preferred embodiments are specified and defined in the subsequent dependent claims.